Uridine, a pyrimidine nucleoside essential for the synthesis of RNA and bio-membranes, has been shown to play a crucial role in modulating the cytotoxic effects of fluoropyrimidines in both normal and tumor tissues. The concentration of uridine in plasma and tissues is tightly regulated at 2-4 MM throughout different species, and uridine homeostasis is maintained by the activity of uridine phosphorylase (UPase) and by intracellular transport mechanisms. UPase is responsible for the reversible phosphorolysis of uridine into uracil, and it is involved in the intracellular activation of the antineoplastic agent 5-fluorouracil (5-FU). We have utilized, both in pre-clinical studies as well as in a Phase I-II clinical trial, inhibition of UPase by its specific inhibitor, 5-benzylacyclouridine (BAU), to elevate uridine concentration in plasma and normal tissues. Modulation of UPase activity resulted in the selective elevation of uridine pools in normal tissues and plasma and in a significant reduction of 5-FU host toxicity. During the past few years, we have uncovered a series of differences between normal and tumor cells in the control and regulation of uridine levels that can be exploited to achieve more selective cytotoxicity in tumors, better protection of normal tissues that are primary targets of fluoropyrimidine toxicity, and improve the therapeutic index of 5-fluorouracil. Our proposal focuses on the role of UPase in regulating the intracellular level of uridine in normal versus tumor tissue and on the assessment of the function of UPase in the activation of 5-FU in tumors compared to normal tissues and as a mediator of host toxicity, specifically at the gastrointestinal tissue level. To achieve these objectives: 1) we will evaluate the role of UPase in the homeostatic regulation of uridine levels in plasma and tissues, utilizing a recently developed UPase knockout murine model and 2) we will analyze the impact of UPase nullification on the activation of 5-FU and its newly developed pro-drugs for their antineoplastic activity and the resulting host toxicities. A second phosphorolytic activity present in mammalian cells, thymidine phosphorylase (TPase), is known to contribute to some of UPase functions, therefore limiting the disruption of uridine homeostasis and participating in the activation of fluoropyrimidines. Thus, a third objective of our project is to generate a second murine model lacking both phosphorolytic activities (UPase and TPase) to define the roles of UPase and TPase in the regulation of uridine homeostasis and fluoropyrimidine activation focusing on the different and possibly complementary functions of these two phosphorylases.